Every mission doesn't have to start from the ground. We don't build a new refinery for every tanker. Make your factory in orbit (or better yet- near the asteroid field). Send the refined metal back.
The deal is, deflect just ONE asteroid to earth orbit, and that's maybe more metal than our civilization has mined so far. The potential is, a new order of society here on earth. The cost - some billions.
"Every mission doesn't have to start from the ground."
I've already addressed that. We do.
"Near the asteroid belt" is meaningless. "The asteroid belt" is huge, and "near" in this case would mean "within a low delta-v orbit". Either way, you're shuttling raw ore or the refining equipment.
There's actually an avenue you haven't proposed: utilizing the asteroid directly for propulsion. There are a few possibilities, including laser ablation (there's a recent PhD thesis on this proposal: http://theses.gla.ac.uk/5219/1/2014GibbingsPhD.pdf ), "pebble drives" in which a mass launcher ejects loose material from the asteroid directly (creating potential collision hazards for other craft, though space is big, really mind-bogglingly big), or solar sails using sunlight to alter orbits gradually over a long period of time.
The deal is, deflect just ONE asteroid to earth orbit, and that's maybe more metal than our civilization has mined so far. The potential is, a new order of society here on earth. The cost - some billions.